worktrees: cleanup some memory leaks

Be sure to clean up looked up references.  Free buffers instead of
merely clearing them.  Use `git__free` instead of `free`.

src/branch.c

@@ -130,14 +130,16 @@ int git_branch_create_from_annotated(
 static int branch_equals(git_repository *repo, const char *path, void *payload)
 {
 	git_reference *branch = (git_reference *) payload;
-	git_reference *head;
-	int equal;
+	git_reference *head = NULL;
+	int equal = 0;
 
 	if (git_reference__read_head(&head, repo, path) < 0 ||
 		git_reference_type(head) != GIT_REF_SYMBOLIC)
-		return 0;
+		goto done;
 
 	equal = !git__strcmp(head->target.symbolic, branch->name);
+
+done:
 	git_reference_free(head);
 	return equal;
 }

src/refs.c

@@ -277,8 +277,8 @@ int git_reference__read_head(
 	}
 
 out:
-	free(name);
-	git_buf_clear(&reference);
+	git__free(name);
+	git_buf_free(&reference);
 
 	return error;
 }

src/repository.c

@@ -2132,7 +2132,8 @@ int git_repository_head_for_worktree(git_reference **out, git_repository *repo,
 out:
 	if (error)
 		git_reference_free(head);
-	git_buf_clear(&path);
+
+	git_buf_free(&path);
 
 	return error;
 }